Generally, since diesel engines can be difficult to start in cold weather, diesel engines are provided with a glow plug as a preheating means for securing startability at an upper portion of a combustion chamber adjacent to an injector. Such a glow plug serves to aid in starting of the diesel engines by heating cold outside air.
FIG. 1 is a front view of a typical glow plug; FIG. 2 is a front view of a unitary metal shell used in a typical glow plug; and FIG. 3 is a reference sectional view showing a state in which a typical glow plug is mounted on an engine.
Referring to FIGS. 1 to 3, a typical glow plug 10a includes a plug's metal shell 100a (hereinafter, “metal shell”) having threads on an upper outer circumferential surface thereof for mounting the glow plug on an engine head 1 and a heating pin 200 disposed to axially pass through the metal shell 100a. 
The heating pin 200 has a heating portion exposed below the metal shell 100a and a terminal disposed above the metal shell 100a, and the metal shell 100a has a through-hole formed therethrough in an axial direction such that the heating pin can be fitted into the through-hole.
The through-hole of the metal shell 100a is formed with a press fitting portion for interference fitting of the heating pin 200.
Such a typical glow plug 10a generates heat when receiving electric power from a battery, thereby heating air or functioning as a hot spot, and some of sprayed fuel directly impinges and evaporates on a hot surface of the glow plug 10a to ignite itself.
However, there are several problems in use of such a typical glow plug 10a for a direct injection type diesel engine.
Recently, as a fuel injection method for diesel engines has been changed from indirect injection type to direct injection type, the glow plug 10a has become longer and thus the overall length of the metal shell 100a has been increased. This causes a problem in that, when the glow plug 10a is mounted on the engine head of the diesel engine, the metal shell 100a suffers from warpage due to compressive load applied to the metal shell.
In addition, increase in overall length of the metal shell 100a is accompanied by deterioration in processability in manufacture of the metal shell.
A glow plug is disposed in a pre-combustion chamber (indirect injection type engine) or in a main combustion chamber (direct injection type engine) depending upon the kind of fuel injection. A glow plug disposed in a pre-combustion chamber, which is a small combustion chamber separated from a main combustion chamber, is shorter than a glow plug disposed in a main combustion chamber. Recently, there is increasing demand for a direct injection type diesel engine.
On the other hand, in order to prevent warpage in mounting the metal shell 100a having a long overall length on an engine, high strength materials may be used to manufacture such a metal shell. However, this causes considerable increase in manufacturing costs of the metal shell 100a as well as difficulty in processing the metal shell 100a. 
A typical metal shell 100a is generally manufactured by cutting or forging. When the metal shell 100a having a long overall length for the glow plug 10a used in a direct injection type diesel engine is manufactured through cutting, a high cutting amount is required, thereby causing significant deterioration in productivity while increasing processing costs.
Particularly, if high strength materials are used to manufacture a metal shell so as to prevent warpage in mounting the metal shell 100a on an engine, this aggravates problems of deterioration in productivity and increase in processing costs as mentioned above.
On the other hand, when the metal shell 100a having a long overall length is manufactured by forging, a larger mold and a thinner punch are required, thereby causing damage to the mold and thus making it difficult to perform forging of the metal shell 100a. Moreover, the metal shell fails to reach desired design strength.
As described above, as the overall length of the metal shell is increased corresponding to the trend of transition from indirect injection type diesel engines to direct injection type diesel engines, an existing metal shell 100 structure raises several problems when employing typical processing methods as mentioned above causing increase in time and costs or deterioration in processability and productivity, which eventually reduces product competitiveness. Therefore, there is a need for an improved metal shell structure.